The present invention relates to a signal detection circuit in a recording and reproducing apparatus which uses a recording medium such as a magnetic tape or a magnetic disk, and more particularly, to a signal detection method and apparatus for a digital data recording and reproducing apparatus which is suitable to detect a high-density recorded signal without error.
In a prior art signal detection circuit for the magnetic recording and reproducing apparatus, data is reproduced by detecting a peak of a magnetic head output waveform. However, data is sprung out or lost by noise and undershoot included in the output waveform. This lowers the reliability of information.
In order to solve the above problem, the following four conditions have been proposed as discrimination conditions for an effective peak, as disclosed in JP-B-60-28460: (1) a detected signal pulse must have a proper polarity and successive Pulses must have polarities thereof reversed; (2) a signal amplitude must exceed a predetermined threshold level; (3) a point at which a polarity of gradient of the signal changes, that is, a peak point must be detected; (4) a voltage change from a maximum amplitude point or the peak point must be the reduction by a predetermined voltage .DELTA.V before a predetermined delay time elapses. When those four conditions are simultaneously met, an effective peak is detected.
FIG. 4 shows a reproduced waveform in which a waveform peak appears at a point other than record data of a logical level "1" or data which is to be data of the logical level "1" has been lost because of a noise included in the reproduced waveform due to a read circuit or a head, or a noise due to a medium defect. Numerals shown over times (n-2) to (n+13) are values sampled by a clock which are synchronized with the peak position of the reproduced waveform. In a signal detection in which a bit-by-bit signal detection by a prior art method is effected to the reproduced waveform, correct signal detection is not attained for the reproduced waveform which has been dynamically changed by data bit length, a so-called "sprung out" waveform in which the recording medium defect, the noise or an invalid peak portion of a threshold value "0" in the transition between data bits fully exceeds the threshold level (a noise sprung-out waveform in which a gradient of the waveform exceeds a predetermined monitoring voltage .DELTA. V), and a lost waveform in which effective data does not exceed the threshold level because of drop-out.
For example, where the reproduced waveform shown in FIG. 4 is detected by the prior art method, the peak values at the times n and (n+2) are lower than the threshold level and they are erroneously detected as the logical level "0" although they are to be detected as the logical level "1". Further, the peak values at the times (n+5) and (n+10) fully exceed the threshold level and they are erroneously detected as the logical level "1" although they are to be detected as the logical level "0". Accordingly, in the reproduction of the high recording density data, the correct data signal detection is not attained.
JP-A-64-86373 discloses an error pointer signal generator which generates an error pointer when a waveform which appears immediately following to a signal to be detected exceeds a predetermined level and is of the same polarity.